Improvement in fabrication processes for semiconductor devices is a continuous goal. One area that may be improved is the ability to equally dope every surface of a device. For example, in some embodiments, a device, disposed on a workpiece, may be a three-dimensional structure, such as a FinFET, a CIS pixel, or a power device trench transistor. In some cases, it may be advantageous to implant an equal amount of dopant into each surface, including the horizontal surfaces and the vertical surfaces. This is referred to as conformal doping. However, some implant processes tend to implant a greater dose into the horizontal surfaces, due to their orientation.
Further, because the ions that strike the horizontal surfaces tend to impact the surface at an angle that is close to normal, these ions tend to have greater energy and penetrate the workpiece more deeply. In contrast, ions that strike vertical surfaces tend to impact those surfaces at an angle that is far from normal, and as a consequence, penetrate to a much lesser extent.
Another area of improvement may be low energy applications. Ultra shallow junctions are becoming more and more common as geometries continue to shrink. However, some semiconductor fabrication machines, such as plasma doping machines, are more effective at higher energies. These higher energies may implant the dopant deeper in the workpiece than desired.
Another area of improvement may be low dose applications. In plasma implantation, the workpiece is exposed to a plasma containing the dopant species to be implanted. The ions are implanted by applying a voltage to the workpiece, but the dopant species is also incorporated into the workpiece by deposition of neutral species on the surface. The neutral species can be in the initial form, for example, B2H6, or can be components (radicals) that are dissociated by the plasma, such as, for example, BH3. The tendency for each type of species to deposit depends on its sticking coefficient, which is a measurable, but often unknown quantity. Deposited species can be incorporated into the substrate by ion bombardment (knock in). In some embodiments, a low dose is desirable and it is desirable to limit the amount of dopant incorporated into the substrate and to prevent surface species from being incorporated. A low dose implant may be achieved through tuning of process knobs, such as, for example, dilution of the doping gas with an inert gas, or by physically blocking the incorporation by a thin sacrificial layer, such as a screen oxide or nitride. Such a layer is used to prevent surface deposition from being mixed into the substrate by the ion bombardment.
Therefore, it would be beneficial if there were a method of conformally doping all of the surfaces of a device. It would be beneficial if this method were also applicable to low energy and low dose applications.